Method for removing dye compounds from an aqueous sample using a functionalized asphaltene

ABSTRACT

A functionalized asphaltene, obtained by refluxing with an acid solution. The functionalized asphaltene contains elevated levels of oxygen content due to nitration and oxidation of the refluxing process. The refluxing process also imparts organic functional groups including at least amines, nitro groups carbonyl groups, carboxylic groups and hydroxyl groups to the functionalized asphaltene, and these functional groups are attached to, thereby coating the surface of a functionalized asphaltene particle. A method for removing dye compounds from an aqueous sample with the functionalized asphaltene is also described.

BACKGROUND OF THE INVENTION

Technical Field

The present invention is directed to modified asphaltenes. The presentinvention includes a process for reacting crude asphaltene with acid toform an acid-functionalized asphaltene. The acid-functionalizedasphaltene has excellent adsorption properties and can be used forremoval of dye compounds from aqueous samples.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Heavy petroleum residue constitutes about 70% of drilled crude oils. Afairly low percent of these residues is being utilized without muchprocessing. Asphaltenes are a class of molecular substances orcomponents found in crude oil, along with resins, aromatic hydrocarbonsand saturates such as alkanes. Although found in insignificantquantities, asphaltenes are nonetheless one of the most notablecomponents present in petroleum due to their precipitation andflocculation properties, which often pose a great challenge towardscracking and refining of crude oil. The tendency of asphaltenes toprecipitate during crude oil recovery can cause severe consequences suchas a sharp decline in oil flow or even blockage of pipelines andprocessing equipment. Asphaltenes can also increase the viscosity ofoil, which can in turn reduce, or even halt, its flow. Furthermore,asphaltenes are known to be coke precursors in acid catalysis and canact as catalyst inhibitors by catalyst deactivation and catalystpoisoning. As such, asphaltenes pose a serious problem to a variety ofprocesses in the petroleum industry.

It has also been reported that asphaltenes are large molecules and arecomposed of highly condensed polyaromatic rings bearing long aliphaticand alicyclic peripheral substituents along with metals and heteroatomsas part of a ring system. Asphaltenes are primarily composed of theelements hydrogen and carbon, with one to three sulfur, oxygen, ornitrogen atoms per molecule. The basic structure is composed of rings,mainly aromatics, with three to ten or more rings per molecule that areusually fused together to form the polycyclic core of the molecule.[Hasan, M.; Siddiqui, M. N.; and Arab, M., Fuel, Volume 67, No. 8,August, 1988, p. 1131; Shirokoff, J. W.; Siddiqui, M. N.; and Ali, M.F., Energy & Fuels, Volume 11, 1997, p. 561—each incorporated herein byreference in its entirety].

Contamination from dyes has attracted tremendous attention owing totheir negative effects on the environment. These toxic pollutants arenonbiodegradable and can accumulate in the human body causing a varietyof diseases and disorders. Dyes such as bromophenol blue and methylorange, widely used in textile and tannery, can cause anemia, insomnia,renal damages, central nervous system damage and dysfunction of theimmune system [G. CamCo-Unal, N. L. B. Pohl, Quantitative Determinationof Heavy Metal Contaminant Complexation by the Carbohydrate PolymerChitin, J. Chem. Eng. Data. 55 (2010) 1117-1121; R. Kiefer, W. H. Höll,Sorption of Heavy Metals onto Selective Ion-Exchange adsorbents withAminophosphonate Functional Groups, Ind. Eng. Chem. 40 (2001) 4570-4576;G. Güçlü, G. Gürda{hacek over (g)}, S. Özgümü

, Competitive removal of heavy metal ions by cellulose graft Copolymers,J. Appl. Polym. SCo. 90 (2003) 2034-2039—each incorporated herein byreference in its entirety]. A variety of techniques like adsorption,precipitation, dialysis, ion exchange, reverse osmosis and extraction,have been reported for the removal of dyes contaminants. One of the mostattractive among these techniques is presumably the adsorption processdue to the availability of different types of efficient adsorbents [K.KesenCo, R. Say, A. Denizli, Removal of heavy metal ions from water byusing poly(ethyleneglyCol dimethacrylate-Co-acrylamide) beads, Eur.Polym. J. 38 (2002) 1443-1448; K. E. Geckeler, Polymer-metal Complexesfor environmental protection. Chemoremediation in the aqueoushomogeneous phase, Pure. Appl. Chem. 73 (2001) 129-136; W. U. Hong, Y.J. In, M. L. Uo, B. I. Shuping, A Simple and Sensitive Flow-InjectionOn-line PreConcentration Coupled with Hydride Generation AtomicFluorescence Spectrometry for the Determination of Ultra-trace Lead inWater, Wine, and Rice, Anal. Chem. 23 (2007) 1109-1112; S. J.Shahtaheri, M. Khadem, F. Golbabaei, A. Rahimi-Froushan, M. R. Ganjali,P. Norouzi, Solid phase extraction for evaluation of occupationalexposure to Pb (II) using XAD-4 sorbent prior to atomic absorptionspectrosCopy, Int. J. Occup. Saf. Ergo. 13(2007) 137-145—eachincorporated herein by reference in its entirety]. Inorganic/organicpolymer hybrid adsorbents have been widely investigated, and theirefficiency of dyes removal has been attributed to the formation of astronger chemical bonding between dye and adsorbent, for instance, aminemotifs in the hybrid materials [Q. Zhang, B. Pan, W. Zhang, B. Pan, Q.Zhang, Arsenate Removal from Aqueous Media by Nanosized Hydrated FerricOxide (HFO)-Loaded Polymeric Sorbents: Effect of HFO Loadings, Ind. Eng.Chem. Res. 47 (2008) 3957-3962; G. P. Kumar, P. A. Kumar, S.Chakraborty, M. Ray, Uptake and desorption of Copper ion usingfunctionalized polymer Coated silica gel in aqueous environment, Sep.Purif. Technol. 57 (2007) 47-56; M. Laatikainen, K. Sirola, E. Paatero,Binding of transition metals by soluble and silica-bound branchedpoly(ethyleneimine). Part 1. Competitive binding equilibria, ColloidSurface A. 296 (2007) 191-205; Y. Tao, L. Ye, J. Pan, Y. Wang, B. Tang,Removal of Pb(II) from aqueous solution on chitosan/TiO₂ hybrid film.,J. Hazard. Mater. 161 (2009) 718-22; Z.-Y. He, H.-L. Nie, C.Branford-White, L.-M. Zhu, Y.-T. Zhou, Y. Zheng, Removal of MO fromaqueous solution by adsorption onto a novel activated nylon-basedmembrane, Bioresour. Technol. 99 (2008) 7954-8—each incorporated hereinby reference in its entirety].

Research on the chemical reactivity of asphaltene has adopted atwo-pronged approach. On one hand, attempts are being made to increaseor decrease the solubility of asphaltene so as to mitigate theflocculation properties of asphaltene and its impact on crude oilviscosity or to increase precipitation of asphaltene and thereby itsseparation from crude oil, respectively. On the other hand, there areongoing efforts in turning asphaltene into a useful material inindustries such as but not limited to polymer and environmentalprotection.

The present disclosure provides a process for chemically modifyingasphaltene to produce a functionalized asphaltene with physicalproperties that are suitable for applications such as but not limited toremoval of pollutant compounds from water by adsorption.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure provides afunctionalized asphaltene comprising 10-35% by weight of elementaloxygen per total weight of the functionalized asphaltene, 3-10% byweight of elemental nitrogen per total weight of the functionalizedasphaltene and 3-10% by weight of elemental sulfur per total weight ofthe functionalized asphaltene. The functionalized asphaltene is obtainedby refluxing a petroleum asphaltene with 70% concentrated nitric acid,and has at least one active group selected from the group consisting ofan amine group, a nitro group, a carbonyl group, a carboxylic group anda hydroxyl group covalently bonded to an asphaltene core.

In certain embodiments, the functionalized asphaltene has an averageparticle size of 10-20 nm.

In certain embodiments, the functionalized asphaltene has a particlesize distribution of 0.5-100 nm wherein at least 60% of the particleshave a particle size of 10-20 nm.

In some embodiments, the functionalized asphaltene has a specificsurface area of no higher than 10 m²/g.

In some embodiments, the functionalized asphaltene has an adsorptionaverage pore width of 4-10 nm.

According to a second aspect, the present disclosure provides a processfor preparing the functionalized asphaltene. The process comprisesrefluxing an asphaltene-acid suspension at 70-90° C. for 1-2 h to formthe functionalized asphaltene. The asphaltene-acid solution comprisesthe petroleum asphaltene and the acid.

In certain embodiments, the acid is a 70% v/v nitric acid.

In certain embodiments, during the refluxing, the asphaltene-acidsolution is agitated at 200-500 rpm.

In certain embodiments, the asphaltene-acid solution has a concentrationof 0.025-0.05 g of the petroleum asphaltene per ml of the acid solution.

In one embodiment, the preparation process further comprises dispersingthe petroleum asphaltene in the acid solution.

In one embodiment, the preparation process further comprises cooling theasphaltene-acid solution after the refluxing, separating and purifyingthe functionalized asphaltene, and drying the functionalized asphaltene.

In one embodiment, the preparation process further comprises pulverizingthe functionalized asphaltene.

According to a third aspect, the present disclosure provides a methodfor removing a dye compound from an aqueous sample with thefunctionalized asphaltene. In the method, the aqueous sample iscontacted with the functionalized asphaltene of claim 1 to adsorb thedye compound onto the functionalized asphaltene.

In some embodiments, the functionalized asphaltene has an adsorptioncapacity of 1-5 mg of the dye compound per g of the functionalizedasphaltene.

In one embodiment, the aqueous sample is contacted with functionalizedasphaltene for 2-7 h.

In another embodiment, the aqueous sample is contacted with 1-10 mg/mlof functionalized asphaltene.

In certain embodiments, the dye compound is bromophenol blue and theaqueous sample is contacted with the functionalized asphaltene at pH4-9.

In alternative embodiments, the dye compound is methyl orange and theaqueous sample is contacted with the functionalized asphaltene at pH of2.5-4 or 8-9.5

In one embodiment, the method further comprises desorbing thebromophenol blue from the functionalized asphaltene at a pH of lowerthan 4 and higher than 9.

In another embodiment, the method further comprises desorbing the methylorange from the functionalized asphaltene at a pH of higher than 4 andlower than 8.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A is an IR spectrum of virgin Arab Heavy asphaltene.

FIG. 1B is an IR spectrum of Arab Heavy asphaltene afterfunctionalization, clearing showing the presence of C═O groups.

FIG. 2A is an SEM image of virgin Arab Heavy asphaltene at 20.0×magnification.

FIG. 2B is an SEM image of virgin Arab Heavy asphaltene at 50.0×magnification.

FIG. 2C is an SEM image of virgin Arab Heavy asphaltene at 2.00×magnification.

FIG. 2D is an SEM image of virgin Arab Heavy asphaltene at 4.00×magnification.

FIG. 3A is an SEM image of functionalized Arab Heavy asphaltene at 20.0×magnification.

FIG. 3B is an SEM image of functionalized Arab Heavy asphaltene at 15.0×magnification.

FIG. 3C is an SEM image of functionalized Arab Heavy asphaltene at 100×magnification.

FIG. 3D is an SEM image of functionalized Arab Heavy asphaltene at 30.0×magnification.

FIG. 4 is a DSC curve of virgin Arab Heavy asphaltene (AH) andfunctionalized Arab Heavy asphaltene (AH075).

FIG. 5A is an EDX spectrum of virgin Arab Heavy asphaltene.

FIG. 5B is an EDX spectrum of functionalized virgin Arab Heavyasphaltene, showing a higher oxygen content.

FIG. 6 is a TGA curve of virgin Arab Heavy asphaltene (AH) andfunctionalized Arab Heavy asphaltene (AH075).

FIG. 7 shows the adsorption kinetic curves of methyl orange (MO) andbromophenol blue (BPB) at 25 ppm in aqueous samples.

FIG. 8 shows the Lagergren second-order kinetic model adsorption curveof MO and BPB on a functionalized asphaltene adsorbent at 295 K.

FIG. 9 shows the effect of initial concentration on the adsorptioncapacity of a functionalized asphaltene adsorbent at pH X for BPB and pHY for MO, for 7 h and at 25° C.

FIG. 10 shows the Langmuir isotherm model adsorption curve of MO and BPBon a functionalized asphaltene adsorbent.

FIG. 11 shows the Freundlich isotherm model adsorption curve of MO andBPB on a functionalized asphaltene adsorbent.

FIG. 12 shows the Temkin isotherm model adsorption curve of MO and BPBon a functionalized asphaltene adsorbent.

FIG. 13 illustrates the pH dependence of dye uptake by a functionalizedasphaltene adsorbent.

FIG. 14 shows the effect of temperature on the adsorption capacity of afunctionalized asphaltene adsorbent.

FIG. 15 is a Vant-Hoff's plot for adsorption of MO and BPB on afunctionalized asphaltene adsorbent.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views.

In the present disclosure, a process for modifying asphaltene where acrude asphaltene derived from crude oil (e.g., a petroleum asphaltene)is reacted with acid is provided. The reaction with the acid at leastpartially oxidizes and/or nitrates the asphaltene and imparts functionalgroups including any of amine, nitro, carbonyl, carboxylate and hydroxylonto the surface of the modified asphaltene product, thereby producing afunctionalized asphaltene.

As used herein, the terms “functionalization” or “surfacefunctionalization” refer to a chemical process that introduces chemicalfunctional groups to a surface of a material, which in the case of thepresent disclosure, is asphaltene.

As used herein, “asphaltene” refers to a class of large molecules thatare found in crude oil that are composed of multiple or polyaromaticrings bearing long aliphatic and alicyclic substituents along with traceamounts of metals such as vanadium and nickel and trace amounts ofheteroatoms such as sulfur, oxygen and nitrogen as part of a ringsystem. Structurally, there are usually 3-10 rings per large asphaltenemolecule, mostly aromatics but not excluding naphtenes and othernon-aromatic cyclic rings, that are fused together to form thepolycyclic core of the molecule. [Hasan, M.; Siddiqui, M. N.; and Arab,M., Fuel, Volume 67, No. 8, August, 1988, p. 1131; Shirokoff, J. W.;Siddiqui, M. N.; and Ali, M. F., Energy & Fuels, Volume 11, 1997, p.561—each incorporated herein by reference in its entirety]. Asphaltenesare insoluble in light paraffins such as n-pentane or n-heptane, butsoluble in aromatic solvents such as toluene.

The asphaltene subjected to the functionalization process of the presentdisclosure, derived from Arab heavy crude, preferably has an AmericanPetroleum Institute (API) gravity of 25.0-30.0. One formula to calculateAPI gravity is from a known specific gravity value of a hydrocarbon:

$\begin{matrix}{{{API}\mspace{14mu}{gravity}} = {\frac{141.5}{SG} - 131.5}} & {{Eq}.\mspace{14mu}(A)}\end{matrix}$

The asphaltene modification or functionalization process of the presentdisclosure includes a step of dispersing crude asphaltene in an acidmixture solution by agitation, preferably by sonication, more preferablysonication at ultrasonic frequencies of >20 kHz for 1-2 h to form ahomogeneous asphaltene-acid suspension having a concentration of0.025-0.05 g of crude asphaltene per ml of the 70% concentrated nitricacid. The asphaltene-acid suspension is then refluxed with simultaneousagitation at 200-500 rpm, preferably 250-350 rpm for 1-2 h, at 70-90° C.After the acid reflux treatment, the asphaltene-acid mixture is cooledto room temperature then the functionalized asphaltene is purified andextracted by repeated cycles of dilution with distilled and deionizedwater, centrifugation and decanting of the supernatant until washingscolor changes from pale yellow to colorless. The functionalizedasphaltene is dried overnight at a temperature of 65-110° C., preferably70-90° C. which afforded 70-75% yield by weight. It is not required tosubject the functionalized asphaltene to other forms of thermaltreatment, such as calcination and sintering, that further physicallyand/or chemically modifies the asphaltene (e.g. phase transformation,densification).

The functionalized asphaltene is then loaded into a planetary ball milland crushed to fine powdered form and used as it is.

In certain embodiments, the functionalization process is preceded by aprocess for extracting crude asphaltene from crude oil. The crude oil isinitially mixed with an alkane solvent (n-heptane) to form a residsolution. The resid solution is heated for 1-3 h, preferably 90-100° C.,then cooled overnight at room temperature, during which the asphaltenecontained in the crude oil will precipitate (as asphaltene is insolublein light paraffin solvents). The precipitated crude asphaltene isseparated from the crude oil by filtration then purified from thefiltrate using toluene (in which asphaltene is soluble). After tolueneis removed from the extracted asphaltene, the asphaltene is washedmultiple times with an alkane solvent to remove maltenes, then dried.The recovered asphaltenes were dried in an oven for about 2 h at 105° C.to obtain a constant weight.

The functionalized asphaltene prepared as described herein has anaverage particle or diameter size of 5-50 nm, preferably 10-40 nm, and aparticle size distribution of 0.5-200 nm, where at least 60% ofparticles fall within the average particle size range, preferably60-70%.

In terms of elemental composition, the changes brought by thefunctionalization process include an increase in nitrogen and/or oxygencontents, preferably both, which are attributed to nitration andoxidation reactions that take place during the functionalization. Asrevealed by energy dispersive X-ray analysis, the functionalizedasphaltene has a nitrogen content of 3-10% by weight, and 3-10% byweight of elemental sulphur per total weight of the functionalizedasphaltene.

As revealed by Fourier transform infrared (FT-IR) analysis, thefunctionalized asphaltene, as a result of nitration and oxidation byconcentrated nitric acid, contains at least type of group such as amine,nitro, carboxyl, carbonyl and hydroxyl functional groups, preferably allsuch groups. The major peaks of FT-IR spectrum of the functionalizedasphaltene, which are lacking in the FT-IR spectrum of the unmodified(virgin) asphaltene, include 3200-3650 cm⁻¹ for O—H and N—H groups,preferably 3350-3600 cm⁻¹, more preferably 3400-3550 cm⁻¹ (i.e. up to88.5% absorbance), 1700-1760 cm⁻¹ for C═O and COOH groups, preferably1715-1745 cm⁻¹ (i.e. up to 86.5% absorbance, 1500-1550 cm⁻¹ for N—Ogroups, preferably 1525-1540 cm⁻¹, (i.e. up to 88.0% absorbance) and1300-1350 cm⁻¹, (up to 90% absorbance, for N—O and C—N groups. Apartfrom the prevalence of these major peaks in the FT-IR spectrum of thefunctionalized asphaltene that correspond to amino, nitro, carbonyl,carboxylic and hydroxyl groups, the functionalization process alsoappears to slightly reduce C—H groups (i.e. peaks at 2800-2950 cm⁻¹,preferably 2850-2930 cm⁻¹) from a peak intensity of 80-90% absorbance to90-98% absorbance., Another peak at 1000-1050 cm⁻¹, preferably 1025-1045cm⁻¹, that corresponds with aliphatic amines, alcohols, carboxylicacids, esters and ethers is present in the FT-IR spectra of both thefunctionalized asphaltene and the unmodified asphaltene, with littlechange to the intensity of the peak.

Scanning electron micrographs of the functionalized asphaltene show thatthe unmodified (virgin) asphaltene particle has a flat and smoothsurface having a root-mean-square roughness value (R_(q)) of lower than0.5 nm. Compared to its unmodified counterpart, the functionalizedasphaltene exhibits an abrasive surface morphology, where the R_(q) is0.5-50 nm, more preferably 5-20 nm. The root-mean-square roughness value(R_(q)) can be calculated according to Eq. (B):

$\begin{matrix}{R_{q} = \sqrt{\frac{1}{n}{\sum\limits_{i = 1}^{n}y_{i}^{2}}}} & {{Eq}.\mspace{14mu}(B)}\end{matrix}$where n is number of sample measurements and Y_(i) is the sum ofabsolute values of surface profile coordinates.

In addition, the functionalized asphaltene particle is dendrimer-like,having ultrafine, nanotube protrusions emanating from the particle coreat a density of 0.5-5 protrusions/nm², preferably 2-3.5 protrusions/nm².These nanotube protrusions have a diameter or width of 0.01-0.25 nm,preferably 0.02-0.1 nm. All of these observations of the surfacemorphology of the functionalized asphaltene, along with the FT-IR, EDX,DSC and TGA characterization results, agree well with the inference thatfunctional groups are covalently bound to the asphaltene core orattached to a surface thereof, thereby coating the surface of afunctionalized asphaltene particle. On the molecular level, the amine,nitro, carboxyl, carbonyl and hydroxyl functional groups are peripheriesattached to the polycyclic core of the asphaltene molecule, along withthe aliphatic and alicyclic peripheral substituents.

The functionalized asphaltene has a Brunauer-Emmett-Teller (BET)specific surface area of no higher than 10 m²/g, preferably 3-6 m²/g.The functionalized asphaltene is also nanoporous, having an adsorptionaverage pore width of the functionalized asphaltene is 1-10 nm (10-100Å), preferably 5-7 nm (50-70 Å).

As indicated by differential scanning calorimetry (DSC), thefunctionalization process increases the combustibility of theasphaltene. As used herein, “combustibility” refers to a measure of howeasily (i.e. at what temperature) an asphaltene sample, which is a fuel,will react with oxygen to produce a high-temperature exothermic redoxchemical reaction. The higher combustibility of the functionalizedasphaltene is attributed to the presence of organic functional groupscoating the surface of the functionalized asphaltene particle. Thecombustion of the functionalized asphaltene, which is an exothermicprocess, can take place at temperatures that are equal or lower than300° C., preferably 100-280° C. The DSC curve of the functionalizedasphaltene, where the heat flow rate is expressed as a function oftemperature, exhibits a prominent and broad peak at 300-600° C.,compared to a small peak at 400-500° C. of the virgin and unmodifiedasphaltene.

Consistent with the increase in combustibility of the functionalizedasphaltene as shown by DSC measurements, thermogravimetric analysesindicate that the functionalization process has reduced the thermalstability of the asphaltene. While the virgin asphaltene decomposes orsuffers from weight loss in a single peak at 400-500° C., the weightlosses for the functionalized asphaltene take place in multiple steps.In one embodiment, the first step with gradual weight loss occurs fromaround 150° C. to around 300° C., more preferably 200-275° C. while thesecond minor decomposition takes places in around 300-400° C.,preferably 300-325° C.

Another embodiment of the present disclosure relates to a method forremoving one or more of a dye compound and a heavy metal ion from anaqueous sample by adsorption with the functionalized asphaltene.Examples of dye compounds that the functionalized asphaltene is capableof adsorbing include but are not limited to methyl violet, malachitegreen, thymol blue, methyl yellow, methyl orange, bromophenol blue,Congo red, methyl purple, bromocresol green, azolitmin, phenol red,methyl red, bromocresol purple, bromothymol blue, neutral red, indigocarmine, naptholphthalein, cresolphthalein, thymolphthalein, cresol redand Alizarine yellow R. In one embodiment, the functionalized asphalteneis capable of adsorbing bromophenol blue and methyl orange.

To remove the one or more dye compounds from an aqueous sample, theaqueous sample is contacted with the functionalized asphaltene, in batchmode, for 2-7 h, When the removal is executed in batch mode, the aqueoussample contains 1-10 mg of the functionalized asphaltene per ml of theaqueous sample, preferably 8-10 mg.

Specifically for bromophenol blue and methyl orange, the functionalizedasphaltene has an adsorption capacity of 1-5 mg of the adsorbate per gof the adsorbent, preferably 1.5-3.0 mg/g.

The adsorption of bromophenol blue and methyl orange by thefunctionalized asphaltene fits well with at least one of Langmuir,Freundlich and Temkin isotherm models, thereby inferring that theadsorption occurs as a monolayer and as a heterogeneous surfaceadsorption.

For the Langmuir isotherm model, separation factor or equilibriumparameter (&) can be used to describe the favorability of adsorption onthe polymer surface. A favorable adsorption is indicated when the R_(L)value is between 0<R_(L)<1, whereas the R_(L) values outside the rangedescribes an unfavorable adsorption. For the adsorption of bromophenolblue and methyl orange, the functionalized asphaltene exhibits aLangmuir separation factor or equilibrium parameter, R_(L), of 0.15-0.8for an initial adsorbent concentration spanning 10-75 mg/dm³. In oneembodiment, the R_(L) values for bromophenol blue and methyl orange are0.3-0.76 mg/g and 0.16-0.59 mg/g, respectively.

The adsorption of bromophenol blue and methyl orange by thefunctionalized asphaltene is also pH-sensitive. The functionalizedasphaltene is contacted to adsorb bromophenol blue from an aqueoussample when the aqueous sample has a pH of 6-7. The adsorption of methylorange by the functionalized asphaltene takes place at pH of 2.5-4. Thismeans that the adsorbed bromophenol blue and methyl orange can be easilydesorbed from the functionalized asphaltene to regenerate the adsorbentat a pH of lower than 4 and higher than 9 for bromophenol blue and a pHof higher than 4 and lower than 8 for methyl orange by treating theasphaltene onto which a dye has been adsorbed with base or acid suchthat the pH conditions encourage release of the adsorbed dye from theadsorbent.

In thermodynamics, the Gibbs free energy (ΔG) is used to describe thespontaneity of a process, and is defined by Gibbs equation:ΔG=ΔH−TΔS  Eq. (B)where ΔH is the enthalpy change, ΔS is the entropy change, T theabsolute temperature and ΔG the Gibbs free energy of the system. Asummary of spontaneity based on the relationship among ΔH, ΔS and ΔG inaccordance with the Gibbs free energy thermodynamic system is given inTable B.

TABLE B Summary of Gibbs free energy thermodynamic system. ΔH ΔS ΔGSpontaneity Positive Positive May be positive or Yes, if T is highnegative, depending on T enough Negative Positive Always negative Alwaysspontaneous Negative Negative May be positive or Yes, if T is lownegative, depending on T enough Negative Negative Always positive Neverspontaneous

The adsorption of bromophenol blue and methyl orange by thefunctionalized asphaltene is also temperature-dependent, and is carriedat 20-60° C., preferably 35-55° C. These adsorption events are alsospontaneous and endothermic, as evidenced by negative ΔG values (Gibbsfree energy) ranging −5 kJ/mol to −20 kJ/mol and positive ΔH values(enthalpy of reaction) ranging +5 kJ/mol to +20 kJ/mol in a temperaturerange of 295-330 K. An increase in the adsorption temperature leads toan increase in randomness or disorder in the system, as revealed bypositive ΔS values (entropy) ranging +5 kJ/mol to +20 kJ/mol in atemperature range of 295-330 K.

The following examples outline various protocols including protocols forseparating asphaltene from crude oil residue, functionalizing theextracted asphaltene with an acid, characterizing the functionalizedasphaltene, removing dye compounds from an aqueous sample by adsorptionwith the functionalized asphaltene, and evaluating adsorption propertiesof the functionalized adsorbent based on various measured or calculatedadsorption parameters.

In these examples, an adsorbent from the functionalization ofasphaltenes was prepared in excellent yield from inexpensive startingmaterials. The adsorbent was found to have an excellent adsorptioncapacity for BPB and MO ions. The adsorption followed Langmuir,Freundlich and Temkin isotherm models as well as Lagergren pseudosecond-order kinetic model. The negative ΔG values and positive ΔHvalues ensured the spontaneity and the endothermic nature of theadsorption process. The excellent adsorption and desorption efficienciesimplied the efficacy of the adsorbent in removing as well as recoveringthe metal ions from aqueous solution. The effective recycling of theadsorbent and its reuse would enable it to be used in the treatment ofcontaminated water in industry.

It is to be understood that the following examples have been includedfor strictly illustrative purposes, and are not intended to limit thescope of the present disclosure.

EXAMPLE 1 Separation of Asphaltene from Crude Oil

First, 7.0 g of Arab Heavy residue or crude oil was transferred to the200 ml beaker and heated with a very small amount of n-heptane in orderto homogenize the solution. This resid solution, after mixing well tobecome homogeneous, was carefully transferred to a 2 L flask and 700 mlof n-heptane was added to the same flask. The flask containing the residsolution was fitted with a mechanical stirrer and placed on the waterbath. The resid solution was heated at 90° C. on the steam bath withcontinuous stirring for about 2 hours in order to maximize thesolubility of the resid in n-heptane. After two hours of mixing, theresid solution covered with aluminum foil was left on the working benchto cool at room temperature for about 24 hours. The long cooling timeproduces efficient precipitation of asphaltenes. The resid solution wasfiltered using a Millipore filtration apparatus with 0.8 μm (37 mm) poresize filter paper. All insoluble material was soxhlet extracted withtoluene and filtered again using same filtering apparatus. The insolublematerial was removed as sludge (coke) and the soluble material,asphaltene, was recovered after evaporating toluene completely. Theasphaltene was collected in a 250 ml beaker and washed several timeswith small portions of n-heptane, in order to remove any traces ofmaltenes, until washings became colorless. The recovered asphalteneswere dried in an oven for about 2 hours at 105° C. to obtain a constantweight. The filtrate, maltene, was recovered by evaporating then-heptane on the steam bath using a rotavapor with continuous blowing ofdry nitrogen until a constant weight of the maltene was obtained.

EXAMPLE 2 Functionalization of Asphaltene

10 g of the asphaltene separated from the Arab Heavy crude oil residuewas dispersed for 1 hour by sonication in a 70% v/v concentrated nitricacid solution. The asphaltene-acid mixture was refluxed while stirringvigorously for 1-2 hours at 70-90 C temperature. After the refluxing,the asphaltene-acid mixture was allowed to cool at room temperature. Thefunctionalized asphaltene was purified by repeated cycles of dilutionwith distilled water, centrifugation and decanting the solutions untilthe pH was approximately 5, in order to extract the residual acids.After the purification step, the functionalized asphaltene was driedovernight in an oven at 100° C. and was pulverized in a ball-mill.

EXAMPLE 3 Characterization of the Functionalized Asphaltene—FourierTransform Infrared Spectroscopy

The virgin Arab Heavy asphaltene and the functionalized asphalteneprepared thereof were characterized by Fourier transform infraredspectroscopy (FT-IR) for an analysis of the organic functional groupspresent in the samples, and the IR spectra of the virgin andfunctionalized asphaltene samples are given in FIGS. 1A and 1B,respectively.

The peaks in the IR spectra were identified based on characteristic IRabsorption frequencies. Based on the analysis and compared to its virginversion, the functionalized asphaltene shows a strong presence of C═Ogroups and also possibly C—N groups. C═O groups are associated withcarboxylic acids, but also possibly esters, aldehdydes and ketones. Theincreased intensity of the peak at 3454.4 cm⁻¹ may be attributed to anincrease in O—H groups as induced by acid oxidation and also newlyformed N—H or amine groups in the functionalized asphaltene. Theformation of amine groups in the functionalized asphaltene is furthersupported by the peak at 1342.1 cm⁻¹ which corresponds to C—N groups inaromatic amines. The C═C aromatic groups in the 1400-1600 cm⁻¹absorption wavelength region do not appear to differ significantlybetween the virgin asphaltene and the functionalized asphaltene, whereasC—H groups in the 2850-3000 cm⁻¹ absorption wavelength region appear tohave reduced in amount in the functionalized asphaltene.

EXAMPLE 4 Characterization of the Functionalized Asphaltene—ScanningElectron Microscopy

Scanning electron microscopy (SEM) images taken of the Arab Heavyasphaltene before (FIGS. 2A-2D) and after (FIGS. 3A-3D) the acidfunctionalization process show that the morphologies of the two samplesare different. The functionalized asphaltene has a relatively rougherand more porous surface, which is attributed to the presence offunctional groups including but not limited to amine, nitro, carboxyl,carbonyl and hydroxyl groups bound to and therefore coating the surfaceof the functionalized asphaltene.

EXAMPLE 6 Characterization of the Functionalized Asphaltene—DifferentialScanning Calorimetry

Differential scanning calorimetry or DSC is a thermoanalytical techniquein which the difference in the amount of heat required to increase thetemperature of a sample and an inert reference material is measured as afunction of temperature. In the present disclosure, the DSC measurementsare carried out in the presence of oxygen or air so that the oxidativestability of the virgin Arab Heavy asphaltene and the functionalizedasphaltene can be determined, and the DSC curves are shown in FIG. 4.

As shown in FIG. 4, the combustion of both the unmodified asphaltene andthe functionalized asphaltene is an exothermic process. The DSC curvefor the functionalized asphaltene exhibits a sharper and higher peak,thereby indicating greater exothermicity which agrees well with thestrong presence of C═O groups and other organic functional groupsindicated by the FT-IR spectra. The functionalized asphaltene alsobegins combustion at a lower temperature compared to the virginasphaltene, i.e. lower than 300° C.

EXAMPLE 7 Characterization of the FunctionalizedAsphaltene—Thermogravimetric Analysis

The thermal stability of the virgin Arab Heavy asphaltene and thefunctionalized asphaltene was determined by thermogravimetric analysis(TGA), and the TGA curves are shown in FIG. 6.

The TGA results, consistent with the DSC results, indicate that thefunctionalization process has reduced the thermal stability of theasphaltene. While the virgin asphaltene decomposes or suffers fromweight loss in a single peak at 400-500° C., the weight losses for thefunctionalized asphaltene take place at two steps. The first step withgradual weight loss from around 150° C. to around 300° C., while thesecond minor decomposition takes places in around 300-400° C.

EXAMPLE 8 Characterization of the FunctionalizedAsphaltene—Brunauer-Emmett-Teller Surface Area Analysis

The Brunauer-Emmett-Teller (BET) surface area of the virgin asphalteneand functionalized asphaltene was analysed and the results aresummarized in Table 2.

TABLE 2 BET surface area and adsorption average pore width data ofasphaltene samples. BET surface area Adsorption average pore widthSample (m²/g) (Å) AH 5.4144 64.4673 AH075 3.8268 52.8895

EXAMPLE 9 Characterization of the FunctionalizedAsphaltene—Energy-Dispersive X-Ray Spectroscopy

The elemental analysis of the virgin and functionalized asphaltenesamples was carried using the energy-dispersive X-ray (EDX) technique,and the results thereof are given in FIGS. 5A, 5B and Table 3. The sharpincrease in oxygen content in the functionalized asphaltene agrees wellwith the strong presence of C═O groups and other organic functionalgroups indicated by the FT-IR spectra. The increase in nitrogen contentalso agrees with the postulated N—H formation in the functionalizedasphaltene in the FT-IR spectra.

TABLE 3 EDX elemental analysis of asphaltene samples. O Sample C H N S(O = 100% − C—H—S) AH 81.28 7.44 1.19 7.17 2.92 AH075 57.08 4.13 5.315.65 27.83

EXAMPLE 10 Dye Adsorption by the Functionalized Asphaltene

The functionalized asphaltene was evaluated for its adsorbent propertiesusing the dye compounds bromophenol blue (BPB) and methyl orange (MO).The BPB, MON and all other reagents were procured from BDH Chemicals andwere of analytical grade. Deionized water used throughout the adsorptioninvestigations.

The adsorption properties of the functionalized asphaltene for BPB andMO ions were determined by spectrophotometric method. The procedure fordye adsorption was as follows: A mixture of functionalized asphaltene(200 mg) in 25 ml of 25 mg/L dye solution was stirred using atemperature-controlled shaker-bath at different pH values (pH=2, 3, 4,5, 6 or 7) for 24 h. The adsorbent was filtered and the filtrate is thenanalyzed by UV-VIS spectrophotometer to find out the amount of dyeremained.

The adsorption capacity (q_(dye) can be calculated using Eq. (1):

$\begin{matrix}{q_{dye} = {\frac{( {C_{i} - C_{e}} )V}{W}{mg}\text{/}g}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where C_(i) and C_(e) are the initial and equilibrium concentrations ofthe dye, respectively, W is the weight of the adsorbent in g and V isthe volume of the solution in milliliter.

Adsorption kinetic studies were carried out by stirring 25 ml of 25 mg/Lsolution in a preferred pH buffer with adsorbent (200 mg) at differenttemperatures and the dye concentrations were determined by taking asmall amount of filtered aliquots at various time intervals. Adsorptionisotherms were constructed by determining the adsorption capacities ofthe adsorbent at different dye concentrations ranging from 10 mg/L to100 mg/L at ambient temperature. Thermodynamic parameters ΔG, ΔH and ΔSwere calculated using data from experiments carried out at differenttemperatures.

A plot of adsorption capacity versus time determines the rate ofadsorption in FIG. 7. It was found that the adsorption equilibrium forBPB and MO ions by adsorbent reached in about 1.5 h. Lagergrenadsorption kinetic model has been reported as a suitable tool toinvestigate the adsorption properties. The following Eq. (3) and Eq. (4)express the linear pseudo first and pseudo second-order kineticequations for the Lagergren model, respectively:

$\begin{matrix}{{\log( {q_{e} - q_{t}} )} = {{\log\; q_{e}} - \frac{k_{1}t}{2.303}}} & {{Eq}.\mspace{14mu}(2)} \\{\frac{t}{q_{t}} = {\frac{1}{k_{2}q_{e}^{2}} + \frac{t}{q_{e}}}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$where k₁ and k₂ are the first-order and second-order rate constant,respectively; q_(t) and q_(e) are the adsorption capacities of the metalions at time t and at equilibrium, respectively. Although BPB and MOboth gave regression value (R²) above 0.9 for the pseudo first-orderLagergren kinetic model, there is a vast difference between theexperimental adsorption capacity and the calculated adsorption capacityso the graph representing the kinetic model has not been displayed. TheMO and BPB were well fitted in the second-order Lagergren kinetic model(FIG. 8) with very close experimental adsorption capacity and thecalculated adsorption capacity (Table 4).

TABLE 4 Lagergren pseudo second-order kinetic model parameters for MOand BPB adsorption at 295K. Lagergren pseudo Calculated Measuredsecond-order q_(e) k₂ q_(e) h^(a) kinetics (mg/g) (h g/mg) (mg/g) (hg/mg) R² BPB 2.334812 0.346769168 1.98378727 1.89035917 0.995 MO2.3668639 1.00397216 2.200404334 5.62429696 0.999 ^(a)Initial adsorptionrate h = k₂ q_(e) ².

The values represented in Table 4 show that the rate constant (k₂) forthe removal of BPB is higher than for MO. However, the functionalizedasphaltene adsorbs a larger amount of MO at the longer periods of time(FIG. 7, Table 4). The adsorption capacity of MO is thus found to belarger than that of BPB. The rationale for such difference could beattributed to the lower effective ionic radii of BPB than that of MO anddifferences in the affinity of phosphonate motifs in the adsorbent forthe dye [J. A. Dean, Lange's Handbook of Chemistry, 15^(th) ed.,McGraw-Hill, 1998—incorporated herein by reference in its entirety]. Theresults revealed that the adsorbent is an efficient adsorbent forremoving both MO and BPB molecules from aqueous solutions.

EXAMPLE 11 Effect of Initial Concentration on the Adsorption of MethylOrange and Bromophenol Blue

The adsorption capacity of the functionalized asphaltene increases withincreasing concentrations of MO and BPB dyes, as shown in FIG. 9.

EXAMPLE 12 Langmuir, Freundlich and Temkin Isotherm Models of MO and BPBAdsorption on the Functionalized Asphaltene

The Langmuir isotherm is based on the assumptions that on structurallyhomogeneous adsorbent, all adsorption sites are energetically equivalentand identical and that the intermolecular force decreases rapidly withdistance. The Langmuir isotherm therefore follows the mechanism as amonolayer adsorption on the surface of the polymer. The Langmuirconstants and adsorption capacities are calculated by linearizedLangmuir isotherm Eq. (4) as follows:

$\begin{matrix}{\frac{C_{e}}{q_{e}} = {\frac{C_{e}}{Q_{m}} - \frac{1}{Q_{m}b}}} & {{Eq}.\mspace{14mu}(4)}\end{matrix}$where q_(e) is milligram of metal adsorbed per gram of the adsorbent;C_(e) is the metal residual concentration in solution at equilibrium,Q_(m) is the maximum specific uptake corresponding to the sitesaturation and b is the ratio of adsorption and desorption rates, theLangmuir constant [A. Cabeza, X. Ouyang, C. V. K. Sharma, M. A. G.Aranda, S. Bruque, A. Clearfield, Complexes Formed between Nitrilotris(methylenephosphonic aCod) and M²⁺ Transition Metals: IsostructuralOrganic—Inorganic Hybrids, Inorg. Chem. 41 (2002) 2325-2333—incorporatedherein by reference in its entirety]. FIG. 10 represents the plot ofC_(e)/q_(e) versus C_(e).

On the other hand, the Freundlich isotherm model describes heterogeneousadsorption systems with uniform energy. The model is expressed by Eq. 5and Eq. 6:

$\begin{matrix}{q_{e} = {k_{f}C_{e}^{l/n}}} & {{Eq}.\mspace{14mu}(5)} \\{{\log\; q_{e}} = {{\log\; k_{f}} + {\frac{1}{n}\log\; C_{e}}}} & {{Eq}.\mspace{14mu}(6)}\end{matrix}$where q_(e) and C_(e) are the equilibrium concentrations of metal ionson the adsorbed and the liquid phase, respectively; k_(f) and nrepresent the Freundlich constants, which can be calculated from theslope and intercept of FIG. 11 which shows the plot of log q_(e) versuslog C_(e).

The Temkin isotherm equation suggests that owing to adsorbent-adsorbateinteractions, the heat of adsorption of molecules in layer decreaseslinearly with coverage, and the adsorption is characterized by a uniformdistribution of the bonding energies. The Temkin isotherm can beexpressed by Eq. 7:

$\begin{matrix}{q_{e = \frac{RT}{b}}{\ln( {A\; C_{e\;}} )}} & {{Eq}.\mspace{14mu}(7)}\end{matrix}$and can be linearized as Eq. (8):q _(e) =B ln A+B ln C _(e)  Eq. (8)where B Corresponds to the adsorption potential of the adsorbent(kJ/mol), A is the Temkin isotherm constant (L/g). The plot of q_(e)versus ln C_(e) of FIG. 12 is used to calculate the Temkin isothermconstants A and B.

FIGS. 10, 11 and 12 illustrate that the adsorption of both MO ad BPBions by adsorbent fitted well the Langmuir, Freundlich and Temkinisotherm models, thereby implying that the adsorption may occur as amonolayer as well as a heterogeneous surface adsorption. The Langmuir,Freundlich and Temkin isotherm model constants are given in Tables 5, 6and 7, respectively.

TABLE 5 Langmuir isotherm model constants for MO and BPB adsorption.Q_(m) B Dye (mmol/g) (dm³/mmol) R² BPB 7.440 0.031 0.95 MO 2.236 0.0690.99

TABLE 6 Freundlich isotherm model constants for MO and BPB adsorption.Dye n k_(f) R² BPB 1.458 0.342 0.995 MO 2.165 0.293 0.943

TABLE 7 Temkin isotherm model constants for MO and BPB adsorption. Dye BA R² BPB 1.3981 0.425 0.953 MO 0.5199 0.579 0.975

For the Langmuir isotherm model, separation factor or equilibriumparameter (R_(L)) can be used to describe the favorability of adsorptionon the polymer surface by Eq. (9):

$\begin{matrix}{R_{L} = \frac{1}{( {1 + {bC}_{0}} )}} & {{Eq}.\mspace{14mu}(9)}\end{matrix}$where C_(o) is the initial dye concentration and b is the Langmuirequilibrium constant. A favorable adsorption is indicated when the R_(L)value is between 0<R_(L)<1, whereas the R_(L) values outside the rangedescribes an unfavorable adsorption [J. Liu, Y. Ma, Y. Zhang, G. Shao,Novel negatively charged hybrids. 3. Removal of BPB from aqueoussolution using zwitterionic hybrid polymers as adsorbent, J. Hazard.Mater. 173 (2010) 438-44—incorporated herein by reference in itsentirety].

The R_(L) values for the adsorption of both dye compounds are given inTable 8, which reveals that R_(L) values fall in the preferred region(0<R_(L)<1). The results thus declare that adsorbent is a promisingadsorbent for the removal of heavy metal ions in aqueous solutions.

TABLE 8 The R_(L) values based on the Langmuir isotherm model. C_(i)R_(L) value (mg/dm³) BPB MO 10 0.7621 0.5913 25 0.5617 0.3666 50 0.39050.2244 75 0.2993 0.1617

EXAMPLE 13 Effect of pH and Temperature on Adsorption

Adsorption experiments were performed at various pH values rangingbetween 2.5-11.0 by using acetate buffer, to find out the effect of pHon uptake of BPB and MO ions. The optimum pH was found to be 6.3 for BPBand 2.7 for MO. pH has a very strong effect on the adsorptioncapacities, as can be seen in FIG. 13. The adsorption of MO at pH 7 isalmost zero while it is 2.20 mg/g at 2.7. Therefore, adsorption of MO bythe functionalized asphaltene has to be done between pH 2-3 whiledesorption will be effective below pH 6-7. Hence, charging anddischarging of metal ions is pH controlled.

Adsorption experiments were also performed to obtain the thermodynamicparameters, and the results are illustrated in FIG. 14. As can be seenfrom the figure, the adsorption capacity increases when the temperatureis increased, suggesting that the adsorption process is endothermic.

A plot of log (q_(e)/C_(e)) versus 1/T is displayed in FIG. 15. Thethermodynamic parameters ΔG, ΔH and ΔS were calculated using Vant-Hoffequation (Eq. (10)), and are tabulated in Table 9 [R. Co

kun, C. Soykan, M. Saçak, Removal of some heavy metal ions from aqueoussolution by adsorption using poly(ethylene terephthalate)-g-itaConicaCod/acrylamide fiber, React. Funct. Polym. 66 (2006) 599-608; A.Ramesh, H. Hasegawa, T. Maki, K. Ueda, Adsorption of inorganic andorganic arsenic from aqueous solutions by polymeric Al/Fe modifiedmontmorillonite, Sep. Purif. Technol. 56 (2007) 90-100—each incorporatedherein by reference in its entirety]. The negative ΔG values ascertainthe spontaneity of the adsorption process.

$\begin{matrix}{{\log( \frac{q_{e}}{C_{e}} )} = {{- \frac{\Delta\; H}{2.303\mspace{14mu}{RT}}} + \frac{\Delta\; S}{2.303\mspace{14mu} R}}} & {{Eq}.\mspace{14mu}(10)}\end{matrix}$

TABLE 9 Thermodyanamic data for BPB and MO adsorption. Temperature ΔG ΔHΔS Dye (K) (kJ/mol) (kJ/mol) (kJ/mol) R² BPB 295 −11.020 +8.509 8.5130.9993 MO 295 −9.166 +6.460 9.173 1.000

As the temperature increases the ΔG values become more negative therebyindicating that the adsorption is more favorable at higher temperatures.Favorable adsorption at higher temperatures is attributed to the greaterswelling of the adsorbent and increased diffusion of metal ions into theadsorbent. The positive values of ΔH certify that the adsorption is anendothermic process. In addition, it can be found in Table 9 that the ΔSvalues are positive, suggesting that the randomness increased duringadsorption of metal ions as a result of release of water molecules fromthe large hydration shells of the metal ions.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. As will be understood by thoseskilled in the art, the present invention may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the disclosure of the presentinvention is intended to be illustrative, but not limiting of the scopeof the invention, as well as other claims. The disclosure, including anyreadily discernible variants of the teachings herein, defines, in part,the scope of the foregoing claim terminology such that no inventivesubject matter is dedicated to the public.

The invention claimed is:
 1. A method for removing a dye compound froman aqueous sample, comprising: contacting the aqueous sample with afunctionalized asphaltene to adsorb the dye compound onto thefunctionalized asphaltene, wherein the functionalized asphaltenecomprises: 10-35% by weight of elemental oxygen per total weight of thefunctionalized asphaltene; 3-10% by weight of elemental nitrogen pertotal weight of the functionalized asphaltene; and 3-10% by weight ofelemental sulfur per total weight of the functionalized asphaltene;wherein the functionalized asphaltene is obtained by refluxing apetroleum asphaltene with an acid, and wherein the functionalizedasphaltene has at least one active group selected from the groupconsisting of an amine group, a nitro group, a carbonyl group, acarboxylic group and a hydroxyl group covalently bonded to an asphaltenecore.
 2. The method of claim 1, wherein the functionalized asphaltenehas an adsorption capacity of 1-5 mg of the dye compound per g of thefunctionalized asphaltene.
 3. The method of claim 1, wherein the aqueoussample is contacted with functionalized asphaltene for 2-7 h.
 4. Themethod of claim 1, wherein the aqueous sample is contacted with 1-10mg/ml of functionalized asphaltene.
 5. The method of claim 1, whereinthe dye compound is bromophenol blue and the aqueous sample is contactedwith the functionalized asphaltene at pH 4-9.
 6. The method of claim 1,wherein the dye compound is methyl orange and the aqueous sample iscontacted with the functionalized asphaltene at pH of 2.5-4.
 7. Themethod of claim 5, further comprising: desorbing the bromophenol bluefrom the functionalized asphaltene at a pH of lower than 4 or higherthan
 9. 8. The method of claim 6, further comprising: desorbing themethyl orange from the functionalized asphaltene at a pH of higher than4 and lower than
 8. 9. The method of claim 1, wherein the functionalizedasphaltene has an average particle size of 10-20 nm.
 10. The method ofclaim 1, wherein the functionalized asphaltene has a particle sizedistribution of 0.5-100 nm with at least 60% of the particles having aparticle size of 10-20 nm.
 11. The method of claim 1, wherein thefunctionalized asphaltene has a specific surface area of no higher than10 m²/g.
 12. The method of claim 1, wherein the functionalizedasphaltene has an adsorption average pore width of 1-10 nm.
 13. Themethod of claim 1, wherein the petroleum asphaltene is refluxed with theacid at 70-90° C. for 1-2 h.
 14. The method of claim 1, wherein the acidis 70% v/v concentrated nitric acid.